Fail-safe differential temperature control system



April 15, 1969 FORBES 3,439,226

FAIL-SAFE DIFFERENTIAL V'IIEMP'ERA'QLIURE- CONTROL SYSTEM we d May 12. 1966 FIG! I coNTRoL Y I To coNTRoLLED SYSIBEM BR l }CIRCUITS CURRENT THRU coIL OF I RELAY RI F|G.2 I F I TIAL CANCELLING RI cuRRENT THRU PULL IN\ coII. OF RELAY DROP ouT' F IG.5

THERMIsToR cuRRENT PLUS DIFFERENTIAL CANCELLING cuRRENT cuRRENT L N TI-IRu THERMISTOR coIL OF [CURRENT RELAY R1 \DROP ouT I F|G.4 DIFF'ERENTIAL CANCELLING cuRRENT cuRRENT FuLL lNg TH Ru K coIL oF RELAY RI DROP OUT /TI-IE MIs R cuRRENT INVENTOR. Norman A. Forbes BY ATTORNEY United States Patent 3,439,226 FAIL-SAFE DIFFERENTIAL TEMPERATURE CONTROL SYSTEM Norman A. Forbes, Louisville, Ky., assignor to American Standard Inc., a corporation of Delaware Filed May 12, 1966, Ser. No. 549,722 Int. Cl. H02h 5/04 US. 'Cl. 317-41 5 Claims ABSTRACT OF THE DISCLOSURE Discloses a fall-safe thermistor sensing circuit for controlling an external circuit in accordance with the changing temperature of a medium into which the thermistor may be immersed. It includes a sensing relay, a rectifierfilter circuit to supply filtered rectified current to the sensing relay winding, another rectifier circuit to supply to the sensing relay winding a pulsing current which is to be additive to the filtered and rectified current, and a thermistor immersed in the medium to be sensed to provide a change in resistance dependent upon the temperature. The current flowing through the sensing relay winding will operate the relay at one temperature of the medium and release the relay at another such temperature of the medium.

This invention relates to temperature control systems and more particularly to those in which a thermistor is utilized as a sensing element, or in a sensing circuit, to sense a temperature which is to be controlled, or to sense a temperature with respect to an external system in which a developed temperature provides and serves as a guide or signal to determine a type of control in accordance with a predetermined schedule or pattern.

An object of the invention is to provide a temperature control system that shall be fail-safe, that is, the opening of a circuit or the short-circuiting of a component in a circuit shall not enable or cause the control system to run away and permit the development of an unsafe or dangerous temperature in a system, or circuit, or apparatus, in which the temperature condition is to be controlled within a predetermined range or pattern.

Another object of the invention is to provide a temperature control system, utilizing a thermistor, in which line voltage variations in the energizing supply circuit shall not cause any appreciable shift in the operating point of the control system corresponding to the cut-out temperature at which the supervised circuit or apparatus is to be subjected to a supervisory operation.

A further object of the invention is to provide a temperature control system utilizing a thermistor, which shall be adjustable to vary the operating point corresponding to the drop-out temperature of the control system, and to vary and particularly reduce the effective differential between the drop-out point and the pull-in point as predetermined by the system, in order that such operating differential may be reduced to a minimum value or increased to a desired permissible normal value in order to follow precisely the operating condition achieved and sought to be controlled, or to open up the operating differential to a value that will still permit the control system to maintain an average operating condition in the circuit or apparatus that is to be supervised by the control system.

Still another object of this invention is to provide a temperature control system utilizing a thermistor, with a simple passive circuit, for variably providing for differential adjustment or differential cancelling.

A still further object of the invention is to provide a temperature control system in which a thermistor is 3,439,226 Patented Apr. 15, 1969 utilized, in which the control system is arranged to operate within a relatively small controllable differential, that permits utilization of the thermistor over a selectable range of the thermistor temperature operating characteristic that has maximum linearity.

Another object of the invention is to provide a temperature control system utilizing a thermistor, that shall permit the operation of the thermistor to be utilized under conditions of minimum thermal resistance between the thermistor and ambient temperatures, in order to stabilize the cut-out temperature point at which the thermistor is to operate.

Briefly, the invention contemplates a control system including a sensing relay, having two normally spaced operating points representing pull-in and drop-out values respectively, which is energized from an alternating current supply source through a diode-capacitor filter combination in series with a sensing thermistor. The realy coil is also energized independently of the thermistor by a current pulse from the same supply source through a variably adjustable resistor and a second diode that is polarized in the same direction as the first-mentioned diode, so the pulse current through the relay coil will be additive with the filtered direct current that is controlled by the resistance of the thermistor. Since those two currents are additive to establish the pull-in condition of the relay coil, the two separate current values, that is, the amplitude of the pulses and the amount of current in the thermistor circuit, may be separately and individually predetermined so long as their sum reaches the value required at pull-in energization of the relay coil.

Other objects, the features and advantages of the invention will be apparent from the following detailed description when read with the accompanying drawing, in which FIGURE 1 is a schematic diagram of a basic system illustrating the invention;

FIGURE 2 is a simple schematic graph illustrating the pull-in and drop-out current values of the thermistor circuit taken alone without modification by the differential adjustment circuit;

FIGURE 3 is a modified graph similar to FIG. 2, showing a superimposed current waveform representing the current for dilferential cancelling;

FIGURE 4 is a modified graph similar to FIG. 2 showing a constant current line representing a thermistor current; and

FIGURE 5 is a modified graph of FIG. 2 illustrating how the thermistor current Waveform in the operating coil of the sensing relay is combined with and modified by the differential adjustment current.

As shown in FIGURE 1, a control system 10 embodying the present invention is shown schematically as including a power supply circuit 20, a thermistor and relay sensing circuit 30, a differential adjustment circuit 40, and a power control circuit 50, which the control system 10 of this invention is intended to supervise and control.

The power supply circuit 20 is shown as an alternating current source 22 of about or volts which energizes the primary Winding 24P of a transformer 24. Operating energy is supplied from the secondary winding 24S thereof to the sensing circuit 30, the differential adjustment circuit 40, and the relay control circuit 50 that is to control the external equipment being supervised.

The sensing circuit 30 starts from the top terminal of the secondary winding 24S and includes a diode D1, a protective fuse F1, the relay coil of a sensing relay R1, and a thermistor T1 to the center tap of the transformer secondary 248 through a return conductor 26. In order to supply substantially continuous direct current to the sensing relay R1, a capactior C1 is con- 3 nected between the anode terminal of the diode D1 and the return conductor 26.

The difierential adjustment circuit 40- includes the diode D1, the fuse F1 and the relay coil of the relay R1, and then continues through conductor 28 through a in the coil drops to or below the value indicated by the lower line, indentified by the legend drop-out.

The dilferential spacing S between drop-out line and the pull-in line, in the graph in FIGURE 2, represents the operating differential of the relay R1, in the absence variable resistor VR and a second diode D2 to the 5 of any other influence on its operation. For the present lower terminal of the secondary winding 248 of the transexplanation, this method of operation may be considered former 24. conventional. The differential involved in the operation The sensing relay R1 has, normally, two operating of the relay represents the differential of the system. points, namely the point at which the relay pulls in Now, in accordance with the present invention, the and the point at which the relay drops out. flhese values graph of FIGURE 3 shows how the relay -R1 may be will vary in any individual relay or in any relay corenergized through the modifying circuit 40 (F lG. 1) by responding to its pre-set setting. Thus, the pull-in point a pulsing current with an amplitude 42 that carries the represents the point at which the current value is such peak of the pulse to a short distance D just under the as to be able to move the relay to the closed position level of the pull-in value of the current through the 6011 from an open or rest position. Correspondingly, when of the relay R1. the current through the operating coil diminishes to a It will be seen from reference to FIGURE 3, that the point which is insuflicent to hold the relay closed, the differential cancelling current indicated by the pulses may relay will drop out. At that point the value of the curstart from a zero level which is less than the drop-out rent in the operating coil constitutes the drop-out value. Valu f Cun nt in th il of 'relay R1. T Zero as The thermistor is a resistive component that has a level of the pulse current may be disposed at any curnegative temperature resistance characteristic. (In other r nt l v l below dropand for convenience of analywords, the resistance of the thermistor varies inversely the base level of that differential Cancelling current with the temperature, that is, as the tempenature in m y be C s d d t0 be th dI P-O t rr n l vel for the thermistor increases the resistance of the thermistor the coil of the relay R1. Thus, the differential cancelling decreases, and as the temperature in the thermistor de Current alone as indicated y the Pulse current of creases, the resistance increases, URE 3, would be insufiicient to cause operation of the That characteristic of the thermistor, which exhibits relay R1 to a change or variation in resistance in accordance with FIGURE 4 is a graph Similar to that Of RE that inverse change i temperature f th h i t but shows the relative value and locus of the thermistor is utilized to control the value of the current through current alone- As there shown, it is Obvious that the the operating coil of th sensing relay, Th h h thermistor current is less than the drop-out value of thermistor T1 is cold, and high in resistance, the curthe relay and far below the Vahle heeessahy cause rent through the coil of relay -R1 is insuflicient to operate P opel'ation'of the Sensing e y R the relay to pull-in position. When the thermistor T1 n FIGURES 1s shownagraph 11 which the d e p is heated sufficiently to lower its resistance and permit Cancelling current and the current through the thermistor more current to flow through the circuit including the are both h e i to illustrate W h r m e hes coil of the relay R1, that relay will pull-in and close its a Value that 18 In eXCeSS the P While for the contact set to provide an appropriate control signal to relay. R a thereby esiahhsheaan p a f th subsequent apparatus i h systhm 40 cond1tion 1n the relay coil to achieve pull-1n operatlon.

Th difl i l or spacing between the 114 vahle Smce the current in the thermistor circuit is essentialof the coil of relay R1 and the drop-out value of the 3! a direct current, Which y for i moment e current to that coil, represents the differential of the slderefii of Constant Value, a pulsing current will be control system. Since that differential between the two esentlauy supenmposed that thermistor current and operating points of the relay represents a correspondi? f the.extent of the 51 f i' ing change in the resistance in the thermistor, as a reerm1tor .current W aciua y not 6 consiant as indicated, in view of the filtering elfect of the diode qulremeilt for operation of the lelay corresponfimg D1 and the capacitor C1, but the variation will be small fihange m the external to effect the thermistor in the slopes of dilferent portions of the current wave. thaiwayi be The illustrations in FIGURE 4 and in FIGURE 5 are, An miportaht feature h the P mventlon'ls the therefore, adequate for a practical illustration of the manner in which such dlfferential 1n the operauon of principle f the invention the relay is modlfied and compressed to a much Smaller The effect of adjusting the differential control circuit desirable Value, y essentially g up some of the 40 by means of the variable resistor VR is indicated in slack in the dilferential spacing, so that only a very small the following table of actual tests:

TABLE I Thermistor Thermistor Thermlstor Thermistor Temperature VR Resistance Resistance Temperature Temperature Differential, (ohms) at Drop-Out at Pull-In at Drop-Out, at Pull-In, F

(ohms) (ohms) F. F.

(co-.-) 1. 520 800 11. 5 20 k. 590 880 10. 1 8k 590 1,040 7.6 6k 1,580 1.160 5.6 4k 1,560 1,500 To+0.5 To -0.2 0.7

change in current, by means of the thermistor variation, The temperature indicated as T0 represents the temwill operate the control system. perature at which the resistance of the thermistor was The manner in Which this feature is aehieved may be the value 1,520 ohms at a condition where the dilterential understood upon referring to FIGURES 2 to 5. adjustment circuit is open. It will be noted from the third As Shown in FIGURE the Value Of e en in the column which shows the thermistor resistor at pull-in,

coil of relay R1, necessary to cause pull-in operation of the relay is indicated by the value represented by the upper line identified by the legend pull in. Once the relay has been pulled in by such operating current value,

that the thermistor does not require as much heating and, therefore, is of higher resistance and operates with a lesser diiferential to achieve pull-in operation of the relay R 1 as a result of the adjusting differential control. Referthe relaywill remain pulled-in or closed, until the current ence to the last column shows how such temperature differential in the thermistor is reduced from a conventional differential of 115 to less than 1, with the use of the invention herein, thereby indicating the effectiveness of the differential adjustment by the auxiliary circuit for introducing the pulsing current into the coil of the relay R1.

The invention has been shown in its simplest aspect by merely indicating the control functions. The operation of the relay R1 completes an operating circuit to a DC operated power relay R2 which is of sufficient capacity to carry power currents for operating heavy external equipment that may be desired. In particular, the operation of relay R1 causes its contact set to close and apply alternating current from the secondary 248 of transformer 24 to the input of bridge rectifier BR. The DC output of bridge rectifier BR is connected to the coil of power or slave relay R2.

A functional operating system has been illustrated in simple form merely to illustrate how the functional features of the system are achieved.

What is claimed is:

1. A temperature control system comprising a relay having a current coil and a mechanism operable by said coil from an initial position to a pull-in position in response to a predetermined pull-in value of current in said coil, and said mechanism being releasable from said pull-in position upon reduction of said coil current to a lower drop-out value;

a current source for supplying an energizing current to said relay current coil;

a thermistor connected in series circuit relation with said relay coil and said source, said thermistor having an operating range of resistance variation between two spaced resistance values corresponding to two spaced internal temperature values, and said thermistor serving by variation of its resistance between said two values to vary the current through said relay current coil to achieve pull-in operation or drop-out operation of said relay, when said thermistor resistance reaches one or the other of said two values;

means controllable by said relay for controlling an external circuit;

and auxiliary circuit means for introducing a calibration-changing current into said relay coil independently of said thermistor, to reduce the normal operating differential of said relay between pull-in and drop-out points, as against operation of said relay alone.

2. A temperature control system, as in claim .1, in which said auxiliary circuit means are electrically connected to said current source to derive energy therefrom to provide the calibration-changing current for the relay coil.

3. A temperature control system comprising:

a relay having a current coil and a mechanism operable by said coil from an initial position to a pull-in position in response to a predetermined pull-in value of current in said coil, and said mechanism being releasable from said pull-in position upon reduction of said coil current to a lower drop-out value;

a current source for supplying an energizing current to said relay current coil;

a thermistor connected in series circuit relation with said relay coil and said source, said thermistor having an operating range of resistance variation between two spaced resistance values corresponding to two spaced internal temperature values, and said thermistor serving by variation of its resistance between said two values to vary the current through said relay current coil to achieve pull-in" operation or drop-out operation of said relay, when said thermistor resistance reaches one or the other of said two values;

means controllable by said relay for controlling an external circuit;

said current source including:

an alternating current supply;

a current rectifier;

and a capacitor connected to be energized from said alternating current supply through said rectifier and, in turn, to supply direct current to said relay coil and thermistor in series;

a differential control circuit containing variable resistance means;

and means connecting said differential control circuit in series circuit relation with said relay coil.

4. A temperature control system comprising:

a relay having a current coil and a mechanism operable by said coil from an initial position to a pull-in position in response to a predetermined pull-in value of current in said coil, and said mechanism being releasable from said pull-in position upon reduction of said coil current to a lower dro -out value;

a current source for supplying an energizing current to said relay current coil;

a thermistor connected in series circuit relation with said relay coil and said source, said thermistor havmg an operating range of resistance variation between two spaced resistance values corresponding to two spaced internal temperature values, and said thermistor serving by variation of its resistance between said two values to vary the current through said relay current coil to achieve pull-in operation or drop-out operation of said relay, when said thermistor resistance reaches one or the other of said two values;

means controllable by said relay for controlling an external circuit;

means for supplying continuous direct current to said relay coil through a circuit including said thermistor;

and circuit means independent of said thermistor circuit for supplying periodic current pulses of the same polarity as said direct current to said relay coil, whereby said current pulses will energize said relay coil to almost pull-in value and thereby reduce the amount of current necessary through the thermistor circuit to energize said relay coil to pullin value.

5. A temperature control system, as in claim 4, in which said means for supplying continuous direct current through said thermistor circuit include a diode and a capacitor combination:

and said circuit means for supplying the periodic current pulses to said relay coil include a diode and a variable resistor.

References Cited UNITED STATES PATENTS 1,872,560 8/1932 Breisky 317132 X 3,014,159 12/1961 Frank 317132 X 3,155,877 11/1964 Jensen 317-41 X FOREIGN PATENTS 873,121 7/1961 Great Britain.

JOHN F. COUCH, Primary Examiner.

W. H. BEHA, 1a., Assistant Examiner.

US. Cl. X.R. 317132. 

